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Entry  Thu Mar 3 07:25:24 2011, kiwamu, Update, Green Locking, screwed up the end PDH box 
    Reply  Thu Dec 12 00:21:04 2013, Koji, Update, Green Locking, X end PDH box oscillation issue solved (Re: screwed up the end PDH box) 
Message ID: 9455     Entry time: Thu Dec 12 00:21:04 2013     In reply to: 4373
Author: Koji 
Type: Update 
Category: Green Locking 
Subject: X end PDH box oscillation issue solved (Re: screwed up the end PDH box) 

What a such long pain we suffered.

After more than three years from Kiwamu's discovery, the PDH box 50kHz oscillation issue was finally solved.

This "weird peak at 50kHz" was caused by the oscillation of the voltage regulator (ON's MC7912).
As it imposed common noise almost everywhere, it screwed up transfer function measurements
like EricQ saw recently.

The negative voltage regulator (79XX) tends to get unstable than the positive counter parts (78XX).

The oscillation was removed by adding 22uF electrolytic capacitor between the output pin (pin3) and the ground pin (pin1) of MC7912.
This is indeed more than 20 times of the specification you can find in the data sheet.

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